This invention relates to a continuously variable speed transmission (CVT) based on a belt drive a having a primary and secondary pulleys each consisting of movable and stationary sheaves and a belt or chain made of a metal or the like wrapped around the pulleys and, more particularly, to a continuously variable speed transmission of this type suitable for use in vehicles. More specifically, this invention relates to a continuously variable speed transmission based on the combination of a belt drive continuously variable speed transmission and a fluid torque converter.
Recently, because of demands for improvements in fuel efficiency, continuously variable speed automatic transmission systems incorporating a belt drive continuously variable speed transmission have attracted attention as transmissions for use in motor vehicles.
The applicant of the present invention had invented an automatic transmission based on the combination of a belt drive continuously variable speed transmission, a fluid torque converter and a forward/reverse rotation changeover mechanism. An example of this transmission is disclosed in Japanese Laid-Open Official Gazette for Patent Application No. 57-129953 (U.S. Pat. No. 4,549,447).
In this continuously variable speed transmission, the movable sheave of each of the primary and secondary pulleys of the belt drive continuously variable speed transmission is operated by a hydraulic actuator, and it is necessary for this hydraulic actuator to constantly apply a predetermined axial force to the pulley in order to maintain a belt clamping force corresponding to a predetermined load torque. The forward/reverse rotation changeover mechanism is used to operate a forward clutch and a reverse brake by means of hydraulic actuators.
The applicant of the present invention had also invented a belt drive continuously variable speed transmission in which axial forces corresponding to transmitted torques are applied to pulleys by means of pressure adjusting cam mechanisms and the effective diameter of each pulley is adjusted by a ball screw mechanism. Examples of this transmission are disclosed in Japanese Laid-Open Official Gazette for Patent Application Nos. 62-159848 and 63-158353.
On the other hand, the fluid torque converter operates in a converter range at the time of starting the vehicle to transmit a torque approximately doubled, thereby ensuring a large load torque necessary at the time of vehicle starting. As the vehicle speed is increased, the fluid torque converter starts operating in a coupling range to transmit a torque substantially equal to that of the engine output shaft. In the continuously variable speed transmission based on the combination of the fluid torque converter and the belt drive continuously variable speed transmission, the torque increased by the fluid torque converter acts on the input shaft of the belt drive continuously variable speed transmission, and it is therefore necessary to increase the transmitted torque capacity of this transmission according to the input torque.
To achieve this, it is necessary to increase the piston area of the hydraulic actuator or to increase the operating hydraulic pressure to increase the axial force produced by the hydraulic actuator. However, the increase in the piston size results in an increase in the overall size of the transmission, which is undesirable in consideration of the intended use in a vehicle. If the operating hydraulic pressure is increased, oil leaks from seals and so on become considerable, resulting in an increase in pump loss and, hence, a reduction in the transmission efficiency as well as deterioration in terms of reliability.
Specifically, the fluid torque converter operates in the converter range for only a short period of time at the time of starting of the vehicle, and it operates in the coupling range for a greater part of the vehicle running time without any substantial increase in the torque. It is disadvantageous to set the transmission torque capacity of the continuously variable speed transmission for the increase in the torque for a short time at the time of vehicle starting, because this setting reduces the total efficiency of the transmission, which result is due to limitations with respect to the strength of the mechanisms, the reliability and the transmission efficiency. A problem of a reduction in the life of the belt is also encountered because a large clamping force is always applied to the belt during operation.
Since as described above the period of time for increasing the torque with the fluid torque converter is short, a method may be adopted in which a line pressure is controlled to increase, in the converter range alone, the operating hydraulic pressure supplied to the hydraulic actuator, or a method of providing a bypass conduction passage functioning in the converter range alone may be adopted. However, it is extremely difficult to control the line pressure or the conduction passage in accordance with the operation of the fluid torque converter which automatically increases the speed from the maximum stall state according to the load torque. To achieve this control, the overall system may be complicated. It is also extremely difficult to suitably control such a factor according to every load torque condition, and there is a possibility of the driver feeling awkward in driving. In a case where the line pressure is controlled, the hydraulic pressure may be set to a higher level in consideration of safety rather than the response to avoid occurrence of slips due to variations in the operating manner. For this reason, the problems relating to the belt durability and the total efficiency have not previously been solved.